Along with the increasing development of science and technology, China has entered an optical fiber broadband and multi-service integrated high-speed developing information era. After integration, the telecommunication network, the TV network and the Internet may bear various infromationization businesses, and can all provide various services such as calling, connecting to the Internet and watching TV for users. This definitely makes greater demands on high bandwidth and flexibility of network infrastructures of data center machine rooms for operators and enterprises, so as to be capable of supporting applications such as high-performance connection, storage area network (SAN), network attached storage (NAS) and high-performance computing (for example, cloud computing). Therefore, in the future, data centers will be dominated by 40 G or even 100 G Ethernets. Especially, in recent years, the concepts such as cloud computing and Internet of Things, and application of VCSEL lasers in multimode optical fiber communications networks make more strict requirements for multimode optical fibers in data centers and central machine rooms, where the optical fiber bandwidth requirement and bend insensitive characteristic of the optical fiber are two most important parameters.
IEEE 802.3ba standard, that is, a 40/100 G Ethernet was approved at Jun. 17, 2010, and the standard supports 150 m multimode optical fiber transmission and 40 km single mode optical fiber transmission at rates of 40 Gb/s and 100 Gb/s. The official release of the standard will certainly accelerate the establishment of 40 G and 100 G Ethernets.
OM3 and OM4 multimode optical fibers are 50 μm-core diameter graded refractive index multimode optical fibers, and have a numerical aperture being 0.200±0.015. The minimum Effective Mode Bandwidth (EMB) of the OM3 and OM4 optical fibers is respectively 2000 MHz·km and 4700 MHz·km. Transmission distances of the OM3/OM4 multimode optical fibers in 10 Gb/s, 40 Gb/s and 100 Gb/s systems are shown in Table 1. It can be seen that, in medium-short distance high-speed network, the multimode optical fibers are well qualified.
TABLE 1Transmission distances of OM3/OM4 multimode opticalfibers in 10 Gb/s, 40 Gb/s and 100 Gb/s systems10 GBASE40 GBASE100 GBASESX (850 nm)SR4 (850 nm)SR10 (850 nm)OM3300 m100 m100 mOM4550 m150 m150 m
Compared with normal OM3/OM4 multimode optical fibers, bend insensitive OM3/OM4 multimode optical fibers have the characteristic of high bandwidth, and further have more excellent macrobending performance, so that they can take more advantages in special deployment conditions such as data centers and central machine rooms; therefore, they gradually become research and development emphasis of various optical fiber and cable manufacturing enterprises, and have the tendency of replacing the normal OM3 and OM4 multimode optical fibers.
An effective method of reducing macro-bending induced loss of an optical fiber is using a design of a trench cladding layer, and when the optical fiber is subjected to small bend, light leaked from a core will be limited in an inner cladding layer in a large proportion and then returned to the core, thereby effectively reducing macro-bending induced losses.
Generally, optimization on an optical fiber with trench-assisted structure generally lies in the structure of the trench cladding layer, that is, the depth and width of the trench cladding layer and a distance from the trench cladding layer to a core layer. To obtain better macro-bending performance, theoretically, greater width and depth of the trench cladding layer can both increase the bend-insensitive performance of the optical fiber, but also inhibits a high-order mode of a multimode optical fiber from leaking to outer-layer pure quartz, thereby affecting the DMD and bandwidth performances of the optical fiber.
Currently, VCSEL light sources of various multimode optical fiber laser manufacturers all have wavelength dispersion of different sizes, and in order to ensure that lasers of different wavelengths of the lasers can transmit synchronously in an optical fiber core layer, considering sensitivities of inner doping of the core layer on transmission rates of lasers with different wavelengths, reasonable doping concentration may be designed to ensure synchronous transmission of a VCSEL light source laser in the optical fiber; especially, when the VCSEL optical fiber performs injection at a central position of the core layer, the central position of the core layer adopts an appropriate Ge/F co-doping manner, thereby effectively optimizing the DMD performance.
Theoretically, to ensure that the multimode optical fiber has desirable DMD and bandwidth performances, accurate control of a core layer refractive index profile of the optical fiber is very important. However, in an actual optical fiber production process, a preform is prepared first, and an optical fiber is then obtained through a drawing process. During fiber forming, glass raw material is inevitably subjected to an external force, so that due to residual stress and component expression, a core layer refractive index profile of the drawn optical fiber will have distortion compared with the original preform. Researches show that, by reasonably designing the viscosity of an inner cladding layer and introducing the design of a functionally graded material, a buffer layer may be formed between the core layer and the recessed layer, to bear a part of the drawing tension, and reduce the influence of the cladding layer interface effect on the core layer. When middle cladding layer of the bend-insensitive multimode optical fiber preform is not doped with F, once it is formed into an optical fiber in a certain drawing tension, a DMD curve graph thereof is generally shown in FIG. 5, that is, shift and widening occur on the core layer at an outer part, which indicates that an outer region of the optical fiber core layer refractive index profile is deformed. When the middle cladding layer is doped with a certain amount of F, under the action of the same drawing tension, a DMD curve graph thereof is shown in FIG. 6, and the distortion of the outer region of the optical fiber core layer refractive index profile is obviously reduced. In a mass production, the appropriate viscosity of the middle cladding layer will keep perfectness of the optical fiber refractive index profile to the maximum extent, thereby improving the yield of target products. However, in the same amount of doped F in the core layer, for different amounts of doped F in the cladding layer, distributions of 850 mm bandwidth and 1300 nm bandwidth are different, as shown in FIG. 7. Therefore, reasonable optical fiber profile structure and composition designed from the perspective of the functionally graded material is conducive to solving the series of problems of deterioration of optical fiber performance parameters caused by differences in structures and components of the core layer and the trench cladding layer in the bend-insensitive multimode optical fiber.
U.S. Pat. No. 8,644,664 describes a bend-insensitive multimode optical fiber structure, in which a bandwidth characteristic of an optical fiber is controlled from the perspective of impurity content change of a core layer; however, a middle cladding layer thereof is not doped with F, but is a Ge-doped glass material, and therefore, viscosity differences from the core layer to the inner cladding layer and to the trench cladding layer are large, and the design of functionally graded material is not used.
Therefore, a heretofore unaddressed need exists in the art to address the aforementioned deficiencies and inadequacies.